1. Field of the Invention
The present invention relates to a complete multi-variable predictive control system for the regulation of single crystal growth processes.
2. The Prior Art
The growth of large single crystals of a specified quality is an important factor in many high-tech applications, such as electronic devices, fiber optics converters, lasers or infrared lenses. While many processes and variants can be used for this purpose, the most common way to grow large crystal boules is to resort to the Czochralski, Float Zone, Horizontal Bridgman, or Vertical Bridgman technique. With regard to the production of crystalline films, epitaxy is the most common technique.
Czochralski Growth
In the Czochralski method, which represents the most frequently used apparatus, a heater (often an ohmic resistor) radiates heat energy towards a crucible containing the molten material, so as to maintain the material above its solidification temperature.
A liner may separate the crucible from the melt. In the case of liquid encapsulated growth, a particular encapsulant covers the melt to prevent evaporation of volatile components. An alternative method for supplying energy to the crucible and the melt, which is frequently used to grow oxide crystals, is to utilize an induction heating system. In this system, a strong alternating current is applied to the coil which generates heat directly within the crucible and/or the melt.
At the outset of the process, a crystal seed is dipped into the liquid and then slowly pulled while a cylindrical crystal is progressively grown. The crystal radiates heat towards the surrounding environment, which consists of the outer shell of the furnace (cooled by water circulation) and, in some cases, of a highly pressurized gas. In this latter situation, the release of heat by conduction and convection into the gas may be important. It is in practice necessary to cause both crystal and crucible to rotate around the vertical axis, in order to ensure axis symmetry of the overall process. This will also control the buoyancy driven flow in the liquid.
The solidification front between crystal and melt is the place where interactions between the liquid and solid phases take place. A major difficulty in the growing of high quality crystals often occurs due to the dependence of interface shape and segregation upon the melt flow pattern. Melt convection can be induced by temperature or solute gradients within the liquid (natural or thermosolutal convection). It can also be induced by surface tension gradient in the liquid-environment interface (Marangoni convection). In semiconductor growth, the Grashof number of the flow is fairly high, which may give rise to an oscillating or even turbulent behavior in the melt. These effects may have severe drawbacks, such as crystal striations. A solution to reduce their importance is to generate a strong magnetic field, which plays a role as a vigorous brake in the flow.
The growth of oxide crystals presents some important differences with respect to semiconductors. While typical Grashof numbers are lower, heat transfer in the melt becomes convection dominated in view of the higher Prandtl number of molten oxides. Another peculiar feature is the importance of internal radiation, which may be a significant mode of heat transfer in the crystal.
Bridgman Growth
An alternative process is to resort to the horizontal or vertical Bridgman technique. Here, the molten material is contained in a cylindrical ampoule, at the bottom of which solidification starts from a crystal seed (after back-melting). The outer thermal environment of the ampoule consists of a hot zone, an insulation and a cold zone. During growth, the ampoule is slowly shifted downwards. The temperature distribution induced by the outer environment in the solid and molten material shifts upwards accordingly, while keeping more or less the same axial profile. Hence, the melt solidifies progressively until the crystal is completely formed.
A major advantage of vertical Bridgman growth is that the solid phase is located under the liquid phase. Consequently, the temperature gradient is pointing essentially upwardly in the melt. If isotherms were exactly horizontal, the layered fluid would be perfectly stable with respect to thermal convection. The problems related to the onset of a buoyancy-driven flow in the melt are, therefore, strongly reduced (a destabilizing radial temperature gradient is however generally present). The situation is more complex when considering solutal convection, which may be important for a non-dilute alloy. Indeed, there is no general rule concerning the dependence of density with respect to solute concentration. There may be solute rejection or solute incorporation at the interface. Hence, the solutal field can have a priori a stabilizing or destabilizing effect. It is worth noting that melt convection outside the diffusion-controlled boundary layer can have a beneficial mixing influence on crystal uniformity.
An efficient way to control the temperature gradient at the outer surface of the crystal and the melt is to replace the classical Bridgman apparatus by a dynamic gradient freeze apparatus. Basically, the heating system consists of a set of controlled annular elements monitored by appropriate thermocouples, which are mostly placed around the periphery of the growth vessel. It is no longer necessary to let the ampoule move downwards in the vessel, since the evolution of the outer temperature profile is achieved by a slow vertical shift of power supply in the heating elements.
Floating Zone Growth
In the Float Zone technique, the material is in the form of a free-standing rod clamped only at its ends, in which a small zone is melted by suitable heating equipment. The melt is suspended like a drop between the two parts of the rod. The molten zone is moved through the rod over its whole length by heater or rod motion. A single crystal can be generated by spontaneous nucleation, or using a single-crystalline seed crystal as the initial part of the rod that is kept unmolten. Most of the time, the zone passage is moved upwardly, because in that case the process has a higher stability. Usually, the growing crystal and sometimes also the melting rod are rotated, mostly with different rotation rates or with counterrotation.
The shape and stability of the molten zone play an important role in floating zone melting. The melt is held against gravity essentially by surface tension. In general, a molten zone becomes more stable the higher its surface tension is, and the lower is its specific weight. Besides, additional forces can act on the melt. In particular, when a high frequency induction heating system is used in semiconductor growth, a skin effect generates non-negligible surface forces which help holding the molten zone up, while increasing melt convection.
Aqueous Solution, Flux, and Hydrothermal Growth
The aqueous solution growth technique can be used to grow several salt single crystals. A simple aqueous solution crystallizer commonly consists of a rotating shaft with perpendicular branches, with a seed crystal attached to each arm. The fluid flow so provided breaks up the boundary layer of rejected solvent, leads to somewhat faster growth, and allows one to obtain more perfect crystals than those grown in quiescent solutions. Supersaturation is maintained by slow cooling, the rates being indeed very slow (0.1.degree. to 1.degree. C. per day). This requires a very careful and reliable temperature control.
The flux growth process in analogous to crystal growth from aqueous solutions, but the solvent solidifies before reaching room temperature. The main advantage of this method is that crystals are grown below the melting temperature. If the material melts incongruently, i.e., decomposes before melting, or exhibits a phase transition below the melting point, one has indeed to look for growth temperatures lower than these phase transitions. Flux growth might thus be used when the melting temperature is very high, and is useful when the vapor pressure at the melting temperature is too high. Thermal strain is minimized, due to the relatively low growth temperature, the very small temperature gradients, and the free growth into a liquid, allowing for the formation of growth facets. The method is very versatile, since a solvent may be found for any material required to be in the form of a single crystal. Flux growth is also suitable for the growth of layers on single crystal substrates. The main disadvantages are the low growth rate and the faceted crystal form. Another disadvantage is the unavoidable presence of ions of the flux as impurities in the crystals, if the solvent contains additional elements. In general, crystal growth from the melt is preferable whenever possible. However, since often only small crystals are needed for basic investigations, the effort might be much smaller than that of melt techniques.
Some very large crystals can also be grown by hydrothermal synthesis. This technique consists in using aqueous solvents or mineralizers under high temperature and pressure, in order to dissolve and recrystallize materials that are relatively insoluble under ordinary conditions. Usually, in hydrothermal growth, dissolution is carried out at the lower hotter zone (nutrient zone) and crystallization is carried out at the upper cooler zone (growth zone). Natural convection currents created by the temperature gradient carry the material from the nutrient zone to the growth zone. Note that there are hydrothermal experiments carried out under extreme pressure conditions (less than 1 bar and greater than 10 kbar).
Physical Vapor Transport Growth
Bulk crystal growth can sometimes be performed by physical vapor transport. The advantage of these techniques is that crystals tend to have a low concentration of point defects, and low dislocation densities, compared to crystals grown from the melt. The reason is that temperatures are usually considerably lower than the melting temperature. Moreover, if the material undergoes a phase transformation, or if it melts incongruently, vapor growth can be the only choice for the growth of single crystals. Compared to melt growth, a disadvantage of vapor growth is however the relatively low growth rate typically encountered.
Several classes of techniques exist for the growth of bulk single crystals from the vapor phase. They are differentiated by the nature of the source material (e.g., whether it is solid or vapor) and by the mechanisms by which it is transported to the growing crystal surface. The conceptually simplest technique is that of sublimation. Here the source material is placed at one end of a sealed tube and heated, so that it sublimes. Then it is transported to the colder region of the tube where it crystallizes. If the source material is a dissociable compound, then the result is dissociative sublimation. Here the transport processes are more complex; and crystal stoichiometry becomes an issue to be considered.
To further control the transport processes, an inert carrier gas can be introduced into the sealed tube. Transport can be facilitated by means of a reversible chemical reaction. Here an active vapor species (often a halogen) reacts with the charge to produce a volatile species (a halide), which is transported to the crystallization zone. There, the reaction is reversed, leading to the deposition of the element or compound to be crystallized with release of the carrier (halogen), which is recycled.
Epitaxy
Thin film deposition processes are related to the bulk growth techniques in many aspects. In summary, one or several very thin layers of material are deposited onto a crystalline substrate to form a layered crystalline film. In epitaxial growth, many methods deal with continuum mechanics, namely liquid and solid phase epitaxy. This includes halogen transport epitaxy, levitation epitaxy and organo-metallic vapor phase epitaxy. All these techniques are very slow, but will produce very high quality products.
Factors Affecting Crystal Quality
Regardless which growth technique is considered, the crystal quality is affected by three factors. These factors include the regularity of the lattice, the presence of impurities in the product, and the inhomogeneity of the crystal stoichiometry.
Lattice defects can take the form of interstitials and vacancies, which are in part generated along the solidification front. Lattice deflects can diffuse and recombine according to complex thermally dependent kinetics. Dislocations or other irregularities, such as twins, grain boundaries, etc. are often induced by the thermal stresses which take place in the crystal near the solid-liquid interface. A key ingredient for obtaining high quality crystals is therefore to control accurately the evolution of the temperature field in the solid during and after growth. This can be achieved by regulating the thermal environment of the growing crystal and, in particular, by using radiative heat shields.
On the other hand, impurities can be incorporated into the crystal during the growth process. This can occur after having been transported through the melt from the surrounding atmosphere and container such as a crucible or an ampoule. This can also occur as a consequence of an imperfect initial composition of the raw material.
The third problem, which can occur when compounds or alloys are grown, is segregation. When several ions or molecules are present in the melt, the composition of the grown crystal can vary axially and radially. This is due to a preferential incorporation of one constituent in the solid phase. Segregation is mostly governed by the melt flow and, to a much lesser extent, by thermal effects. This is because the melting temperature can depend on crystal and melt composition. Obtaining a uniform crystal composition is of course a key objective when e.g., Periodic Table Group III-V compounds (such as GaAs crystals) are grown.
Although any growth process involves peculiar difficulties, these are in general quite similar in nature and can be related to the three main quality factors hitherto described.
The preferred objective considering the Czochralski process (Cz process) is to produce crystals whose diameter is almost constant and whose quality is the highest possible. This is a difficult objective, since most relevant physical quantities, such as interface deflection, growth speed of the lattice, etc., which characterize crystal quality, cannot at all be measured during growth. This is due to (1) the high temperatures prevailing during the process; (2) the extreme sensitivity of the crystal lattice with respect to any change to the environment of the liquid-solid interface; (3) the inherent batch nature of the process; and (4) the high sensitivity to any contamination.
Silicon Growth
In particular, the quality of silicon crystals is specified according to the requirements of the end user. Such specifications for the bulk quality include the concentration of oxygen, the resistivity, the density of micro defects and the maximum concentrations of metallic impurities, e.g., Fe, Cu. A case of particular interest is the problem of oxygen incorporation in silicon crystals grown by the Czochralski process. As only quartz crucibles can be used, the solid SiO.sub.2 is dissolved partly into the melt and is evaporated mostly from the melt-gas interface. A fraction of the oxygen remains however dissolved in the liquid and arrives at the crystal-melt interface, where a significant amount is incorporated into the solid. The presence of oxygen in silicon crystals gives rise to positive and negative quality aspects. Oxygen precipitates (BMD) in the center of the wafer getter metallic impurities during device manufacturing, which reduces current leakage. The dissolved oxygen in the wafer hardens the silicon lattice and makes it less susceptible for geometrical deformations. On the other side, BMD are detrimental for the devices, if they are located near the wafer surface. In this respect, the objective is to obtain a very homogenous and reproducible oxygen content in the crystal within tight specifications.
The concentration of dopants, such as B, P, As, Sb, determines the resistivity of the wafers. In order to obtain a high yield in device manufacturing, tightly specified ranges of dopant concentrations have to be incorporated homogeneously into the crystal.
However, during growth, the concentration of oxygen and dopants are non-measurable parameters, which significantly determine the quality of silicon crystals. Usually, the optimization of oxygen and dopant incorporation into the crystal is done by trial and error. Therefore, the development of crystal growth processes is very time consuming and expensive.
During growth, so-called as-grown defects form, including vacancies, self-interstitials, oxygen and other impurity atoms, which diffuse and react with each other according to complex thermally dependent kinetics. Thus, a key ingredient for obtaining high quality crystals is again to control accurately the evolution of the temperature field in the solid during and after growth. This can be achieved by regulating the thermal environment of the growing crystal. In Czochralski silicon growth, peculiar components act as radiative heat shields. The pull rate is selected in such a way that the thermal history of the crystal fits with the best experimental results.
Measurement Techniques
Temperature measurements are very difficult to perform at high temperatures, since thermocouples can easily experience fast degradation, loss of accuracy etc. Pyrometers are in principle less accurate and are more difficult to calibrate. Any on-line intrusive measurement in the melt (or the gas in physical vapor growth) is strongly prohibited, since the melt (or gas) composition cannot at all be modified during growth. This is due to the severe composition requirements of most single crystals in high-tech applications.
Possible Control Strategies
A single control strategy is not at all feasible, since the system geometry can undergo quite important changes during full crystal growth. In particular, the Cz process can be subdivided in several stages: melt down and stabilization, seeding, cone growth, roll-over or shouldering, body growth, tail-end growth and after-heating. An optimal control structure would therefore require the use of a system of controllers that can continually adapt to the evolution of the geometry. This would be dictated by the crystal length increase and the melt volume decrease. Besides these difficulties, the growth control strategy is however facilitated in most cases by the process slowness. (Growing semi-conductor crystals can take several hours, or even a full day. Growing oxide crystals can take several days, or even weeks.)
Off-Line Control
In Czochralski growth, present control devices are able to control the diameter or the weight of the solidified crystal. These are the only easily measurable quantities, and the control device acts on the pull rate and the heater power. No on-line quality control is presently available, and quality can only be controlled by off-line numerical simulations (with a view to adapting the processing conditions). Finite element codes, such as `FEMAG`, are devoted to performing off-line simulations (F. Dupret, P. Nicodeme, Y. Ryckmans, P. Wouters, M. J. Crochet, Int. J. Heat Mass Transfer, 33 (1990) 1849; M. J. Crochet, F. Dupret, Y. Ryckmans, F. T. Geyling, E. M. Monberg, J. Cryst. Growth, 97 (1989) 173; F. Dupret and N. van den Bogaert, in: Handbook of Crystal Growth, Vol. 2b, Chapter 15, edited by D. T. J. Hurle, North, Holland, (1994); R. Assaker, N. van den Bogaert, F. Dupret, "Dynamic Global Simulation of Bulk Crystal Growth Under the Influence of an Axisymmetric Magnetic Field", Proceedings of the Second International Conference on Energy Transfer in Magnetohydrodynamic Flows, Aussois, France, 1994). These simulations are employed to predict the temperature distribution and the shape of the free boundaries in all parts of a crystal puller. This includes the crystal, the melt, the crucible, the graphite susceptor, the heater and several insulators as a function of time, if necessary. The reference discusses in detail the underlying physical and numerical principles of the finite element code `FEMAG`, which are summarized in this section. Based on the assumption of an axial symmetric furnace geometry, the code takes into account heat transfer by conduction in solid parts (e.g. crystal, heater, insulation) and radiation in the furnace enclosures. Convective heat transfer in the melt is computed by solving the complete Navier-Stokes equations with appropriate turbulence models. To reduce the computation time, it is possible as well to approximate the convective heat transfer in the melt by means of an enhanced equivalent thermal conductivity. Heat transport by inert gas convection is usually neglected in silicon growth due to low pressure (ca. 20 mbar). This effect can be modelled approximately, when necessary.
The shape of the solid/liquid interface, which is a priori unknown, is determined by the heat flux balance in the vicinity of the interface. That is, the heat flux q.sub.m from the melt to the interface and the heat generated by solidification q.sub.f is equal to the heat flux q.sub.c into the crystal. Additionally, the temperature at the solid/liquid interface T.sub.sl is presumed to be equal to the melting temperature T.sub.m.
In the furnace enclosures, heat is transferred by radiation, which couples the heat fluxes that are emitted, absorbed and reflected by solid and liquid surfaces. While all surfaces are usually assumed to be gray emitters, a band energy approach can be used to model wavelength dependent properties. This is necessary for semi-transparent materials (such as encapsulant layers or quartz tubes). These are modelled as transparent in a given range of frequencies and as opaque in the remainder of the spectrum. Diffuse radiation is taken into account, while specular radiation is neglected. A modified Gebhard method was applied to calculate the incoming and outgoing heat fluxes from the surfaces. View factors are computed by taking viewed and hidden surfaces into account.
The heat transfer equations form a set of non-linear, coupled, axisymmetric two-dimensional partial-differential equations, which are solved numerically by the finite element method. Galerkin's method is applied for space discretization and the resulting set of non-linear algebraic equations is solved by Newton's method.
Data of the physical properties of all materials used in the simulations are required and can be selected from literature or from the suppliers of the consumables. Thermal boundary conditions at the furnace walls have to be defined, e.g. 320.degree. K at the furnace walls.
Present Control Strategies In Silicon Growth
Each stage of the growth is generally controlled through a different strategy. The body phase regulation can include a system with two controllers. One controller will regulate the diameter, on the one hand by acting on the pull rate through a PD controller. The other controller will regulate the average pull rate, on the other hand by acting on the heater power through a PI controller. This regulation strategy for body growth is most often satisfactory with regard to the required set points for diameter or pull rate. However, control of the body phase for very large crystals is likely to represent a new difficulty. This is because the process during the body phase can no longer be considered as stationary, and might become unstable due to large time constants and response times of the power controller.
The difficulties of existing control strategies mainly deal with the regulation of the other stages of the process. During seeding and cone growth, only partial control is achieved. In particular, a predetermined heater power evolution can be applied, while diameter is controlled via the pull rate. Control of the stages of melt down, roll-over and tail-end is usually achieved by applying off-line precalculated set-point curves for heater power and pull rate. The control of the tail-end stage is a particularly difficult problem, since diameter cannot be measured today at this stage of growth.
Other drawbacks of the current control strategies are that crystal quality is not directly addressed on-line, but only indirectly through actions on the pull rate or the diameter.